1. Field of the Invention
The present invention relates to a frequency converter circuit provided to a tuner contained in a cable modem (referred to as "cable modem tuner" hereinafter) which is necessary for high-speed data communication utilizing any vacant channel of the cable television (referred to as "CATV" hereinafter). More particularly, the present invention relates to a frequency converter circuit applied to a cable modem tuner that can properly maintain the characteristics of high-frequency signals.
2. Description of the Background Art
The CATV is now introducing HFC (abbreviation for Hybrid Fiber/Coax) constituted by employing the coaxial cable as the house drop line and employing the optical fiber as the trunk line network. The HFC is employed for providing data communication service with a broad band of several M bits per second to each domestic site. A high-speed data line of the transmission rate of 30 M bits/second with a bandwidth of 6 MHz can be implemented even by the 64 QAM (abbreviation for Quadrature Amplitude Modulation) which is not regarded as high technology now. By utilizing any vacant channel of CATV, high-speed data communication of 4 M bits/sec to 27 M bits/sec is possible. In order to realize such high-speed data communication, a cable modem is required that has a main function of converting transmission signals to those signals used by domestic data processing equipment and vice versa.
FIG. 6 is a block diagram of a conventional cable modem tuner 100. Referring to FIG. 6, cable modem tuner 100 includes an input terminal IN for connecting a CATV station 300 with cable modem tuner 100 via a cable 400 for communication, a data terminal D for inputting data signals modulated by the quadrature phase-shift keying by an external QPSK (Quadrature Phase-Shift Keying) modulator 500, an output terminal OUT for outputting processed IF (abbreviation for Intermediate Frequency) signals, an IF filter 101, a switching circuit 102, first tuning circuits 103-105, high-frequency amplifier circuits 106-108, second tuning circuits 109-111, a VHF.HIGH/LOW switching circuit 112, frequency converter circuits 113 and 114, a PLL (abbreviation for Phase-Locked Loop) channel select circuit 115, an IF first amplifier circuit 116, an SAW filter 117, an IF second amplifier circuit 118, and an upstream circuit 119.
As for CATV signals related to the cable modem tuner, generally upstream signals (signals transmitted from the cable modem tuner to the CATV station) are operated at 5-42 MHz, and downstream signals (signals transmitted from the CATV station to the cable modem tuner) are operated at 54-860 MHz.
In operation, a data signal applied via data input terminal D is transmitted via upstream circuit 119 to be output from input terminal IN toward CATV station 300. Upstream circuit 119 is a low-pass filter having the passband of 5-42 MHz.
A signal transmitted from CATV station 300 is input to the cable modem (cable modem tuner 100) via input terminal IN. Processing of the downstream signals in cable modem tuner 100 is hereinafter described.
A downstream signal supplied from input terminal IN is passed through IF filter 101, and transmitted, by switching circuit 102, to any one of circuitry 201 for receiving a UHF band (470-860 MHz) (hereinafter referred to as "UHF band circuit"), circuitry 202 for receiving a VHF.HIGH band (170-470 MHz) (hereinafter referred to as "VHF.HIGH band circuit"), and circuitry 203 for receiving a VHF.LOW band (54-170 MHz) (hereinafter referred to as "VHF.LOW band circuit") (i.e. to any band circuit corresponding to a desired reception channel). IF filter 101 is a high-pass filter having the attenuation band of 5-46 MHz and the passing band of 54 MHz and above.
One of UHF band circuit 201, VHF.HIGH band circuit 202, and VHF.LOW band circuit 203 is selected according to a desired reception channel. Only the selected band circuit is in an operating state, and other band circuits that are not selected are in a non-operating state. For example, when a channel of the UHF band is received, UHF band circuit 201, i.e., first tuning circuit 103, high-frequency amplifier circuit 106, second tuning circuit 109, and frequency converter circuit 113 are in the operating state, and VHF.HIGH band circuit 202 and VHF.LOW band circuit 203, i.e., first tuning circuits 104 and 105, high-frequency amplifier circuits 107 and 108, second tuning circuits 110 and 111, VHF.HIGH/LOW switching circuit 112, and frequency converter circuit 114 are in the non-operating state.
Those circuits common to respective band circuits such as IF filter 101, switching circuit 102, PLL channel select circuit 115, IF first amplifier circuit 116, SAW filter 117, IF second amplifier circuit 118, and upstream circuit 119 are always in the operating state. VHF.HIGH/LOW switching circuit 112 and frequency converter circuit 114 are common to VHF.HIGH band circuit 202 and VHF.LOW band circuit 203, and these circuits are in the non-operating state only when a channel of the UHF band is received.
An operation in UHF band circuit 201, VHF.HIGH band circuit 202 and VHF.LOW band circuit 203 is now described. A signal received via input terminal IN, IF filter 101 and switching circuit 102 is supplied to one of the first tuning circuits 103-105. The received signal is output from the one of the first tuning circuits 103-105 as a signal of a desired frequency, amplified by a corresponding one of high-frequency amplifier circuits 106-108 in the next stage, and thereafter supplied to a corresponding one of the second tuning circuits 109-111. The supplied signal is input to the corresponding one of the second tuning circuits 109-111 and output therefrom again as a signal of the desired frequency to be supplied to a corresponding one of frequency converter circuits 113 and 114.
Frequency converter circuits 113 and 114 are respectively constituted of respective mixer circuits 51 and 61 and respective local oscillation circuits 52 and 62. From the corresponding one of frequency converter circuits 113 and 114, an IF signal obtained by mixing the high frequency signal supplied from the corresponding one of the second tuning circuits 109-111 with an oscillation signal from a corresponding one of local oscillation circuits 52 and 62 using a corresponding one of mixer circuits 51 and 61 is output. In other words, the high-frequency signal output from second tuning circuits 109-111 is converted to the IF signal by frequency converter circuits 113 and 114. The oscillation frequency of local oscillation circuits 52 and 62 each is controlled by PLL channel select circuit 115. Frequency converter circuits 113 and 114, and PLL channel select circuit 115 are formed as an IC of one chip.
The IF signal is thus output from the corresponding one of UHF band circuit 201, VHF.HIGH band circuit 202 and VHF.LOW band circuit 203, amplified by IF first amplifier circuit 116, then amplified again by IF second amplifier circuit 118 via SAW filter 117, and output from output terminal OUT.
FIG. 7 shows a conventional circuit structure of frequency converter circuit 114 illustrated in FIG. 6. In cable modem tuner 100, frequency converter circuit 114 which is common to VHF.HIGH band circuit 202 and VHF.LOW band circuit 203 has the structure illustrated in FIG. 7. Referring to FIG. 7, an IC13 is connected to a resonant circuit 41 connecting terminals T5-T7 via feedback capacitors 14-17 that are described in the following.
IC13 includes a differential amplifier circuit 42, mixer circuits 1 and 2 that constitute mixer circuit 61, buffer amplifiers 3 and 4, terminals T1 and T2 for input and terminals T3 and T4 for output to and from mixer circuits 1 and 2, and a terminal T8 to apply power supply voltage VCC.
Differential amplifier circuit 42 includes transistors 51 and 52 that constitute a differential stage, bias resistors 7, 8, 10 and 11, collector resistors 6 and 12, and an emitter resistor 9.
Power supply voltage VCC applied to terminal T8 is divided by bias resistors 7 and 8 and bias resistors 10 and 11 to be supplied to the bases of transistors 51 and 52 respectively that constitute the differential stage. Power supply voltage VCC applied to terminal T8 is also supplied to the collectors of transistors 51 and 52 via collector resistors 6 and 12 respectively. The emitters of transistors 51 and 52 are commonly grounded via emitter resistor 9. The bases of transistors 51 and 52 are respectively connected to the input stages of respective mixer circuits 1 and 2 via buffer amplifiers 3 and 4.
Resonant circuit 41 and differential amplifier circuit 42 within IC13 are connected as follows. One end of resonant circuit 41 is connected to the base of transistor 51 via feedback capacitor 14, and to the collector of transistor 52 via feedback capacitor 15. The other end of resonant circuit 41 is connected to the collector of transistor 51 via feedback capacitor 16, and to the base of transistor 52 via feedback capacitor 17.
In operation, change of the bias voltage upon application of the supply voltage causes signals oscillating at the resonance frequency (100-500 MHz) of resonant circuit 41 because of a balanced oscillate operation in local oscillation circuit 62 to be applied to mixer circuits 1 and 2 in reversed phases relative to each other. Accordingly, high-frequency signals of VHF supplied from VHF.HIGH/LOW switching circuit 112 via terminals T1 and T2 are converted to IF signals by mixer circuits 1 and 2 to be output from terminals T3 and T4 respectively. The IF signals output from terminals T3 and T4 respectively are combined, with one of the IF signals having a reversed phase relative to the other IF signal, to be supplied to IF first amplifier circuit 116 shown in FIG. 6. VHF.HIGH/LOW switching circuit 112 allows high-frequency signals output from second tuning circuit 110 to be applied to terminals T1 and T2 with phases reversed relative to each other when the VHF.HIGH band is received, and allows high-frequency signals output from second tuning circuit 111 to be applied to terminals T1 and T2 with phases reversed relative to each other when the VHF.LOW band is received.
The resonance frequency of resonant circuit 41 is variably set by tuning voltage applied to terminal T5. When the VHF.HIGH band is received, voltages of a high level and a low level are applied respectively to terminals T6 and T7. When the VHF.LOW band is received, voltages of the low level and the high level are respectively applied to terminals T6 and T7. Accordingly, the variable band of the resonance frequency is changed depending on the VHF.HIGH band and VHF.LOW band.
In cable modem tuner 100, the high-frequency signals supplied via first tuning circuits 103-105, high-frequency amplifier circuits 106-108, and second tuning circuits 109-111 are frequency-converted to IF signals by frequency converter circuits 113 and 114 as described above. In this case, it is desirable that noises generated in the frequency-converting operation are restricted as much as possible. In particular, when local oscillation circuits 52 and 62 are controlled by the PLL, the phase noise generated from local oscillation circuits 52 and 62 respectively deteriorates C/N ratio (Carrier to Noise ratio).
However, in the conventional structure, local oscillation circuit 62 performs the balanced oscillate operation and the feedback capacitors are connected to the collector sides of respective transistors 51 and 52 constituting the differential stage in frequency converter circuit 114 which is common to VHF.HIGH band circuit 202 and VHF.LOW band circuit 203 as illustrated in FIG. 7, so that the collectors of transistors 51 and 52 are inductive for the high-frequency signals. Therefore, when capacitive elements are connected to the collector sides of transistors 51 and 52, a parasitic series resonant circuit is formed, resulting in the so-called trap phenomenon. The trap phenomenon is remarkable especially in the frequency band of the VHF.HIGH band. The parasitic series resonant circuit formed in the trap phenomenon absorbs the power for oscillation, so that oscillation output decreases and the phase noise increases. For this reason, the quality in reception of the VHF.HIGH band is not fine in the conventional cable modem tuner 100.
In addition, the circuit performing the balanced oscillate operation requires a large oscillation power and the local leakage related to an oscillation signal increases. Further, since transistors 51 and 52 constituting the differential stage are required to be formed as the complementarily symmetric type transistors, the stability in oscillation is unsatisfactory. The large number of components associated with the oscillation circuit leads to the poor economy.